Is Video RAM a Good Swap Device?

sean4u writes "I use a 'lucky' (inexplicably still working) headless desktop PC to serve pages for a low-volume e-commerce site. I came across a gentoo-wiki.com page and this linuxnews.pl page that suggested the interesting possibility of using the Video RAM of the built-in video adapter as a swap device or RAM disk. The instructions worked a treat, but I'm curious as to how good a substitute this can be for swap space on disk. In my (amateurish) test, hdparm -t tells me the Video RAM block device is 3 times slower than the aging disk I currently use. If you've used this technique, what performance do you get? Is the poor performance report from hdparm a feature of the hardware, or the Memory Technology Device driver? What do you use to measure swap performance?"

Comments

AGP or PCI-Express

[j_sp_r]

Is your adapter an AGP or PCI-Express card? Because PCI-Express has fast lanes both ways, and AGP is not so fast in writing back. That could explain a part of the performance problems.

Re:AGP or PCI-Express

[Andy Dodd]

Whether it is built into the motherboard or is built into the chipset itself, it almost always still has an "internal only" PCI, AGP, or PCI-E interface, and is thus affected by the performance limitations of that interface.

Probably a good idea, provided you have PCIe

[default luser]

I'm assuming your ancient system uses an AGP interface for graphics, which has a very fast download rate, but very poor upload. The maximum performance of AGP uploading data from the card memory to the rest of the machine is pretty slow (less than 100MB/sec, IIRC), and it will vary depending on the implementation. This is probably the reason you got such slow benches.

PCIe will likely give you performance more in-line with main memory (most implementations now are hitting 1-2 GB/s).

Re:Probably a good idea, provided you have PCIe

[dougmc]

ATA 133 (I will assume this, due to the "aging hd mentioned) is only 17MB/sec for comparison's sake. No, ATA 133 is theoretically 133 MB/s. It's bytes, not bits.

And I used to regularly get sustained 25-30 MB/s from single drives (40 GB or so) on ATA 33 interfaces. Going to ATA 66, 100 or 133 may increase the speed when hitting the on-drive cache, but the drives themselves usually can't go that fast. How fast are the fastest IDE drives nowadays for sustained, sequential transfers -- 50 MB/s or so?

Re:Probably a good idea, provided you have PCIe

[ElecCham]

(ObDisclaimer: I work for Seagate.)

On a current-model 7200RPM SATA drive, you can expect to see around 80MB/sec at the outer edge of the disk. And the rule of thumb is, you see half that at the inner edge, and three-quarters in the middle. So call it a (nearly) guaranteed 40MB/sec, and an average of 60MB/sec.

These are not hard-and-fast numbers, but it's a pretty good estimate for a modern drive.

size

[TheSHAD0W]

How much RAM is in your video card? 64 megabytes? 128? If it's an older machine, probably much less than that. Assuming you have less than a gigabyte of main RAM in your system it's probably much more worthwhile to drop a few dollars on expanding that and running whatever RAM disk you need in there.

Re:size

[arivanov]

Not quite so.

If it is really old it may be running one one of the early Intel Pentium Triton chipsets. The TX will not cache any memory above 64 and the HX needs to be reconfigured to cache above 64. Even after reconfiguration it will just about work for 512MB. There are other similar vagaries related to most old hardware. Ali depending on release and version tanks at 384 or 768 and so on. Even chipsets as recent as Intel 815e while capable of 2G were deliberately bastardised to support only 512MB in order not to undercut the inexistent market for high-end Rambus/i810 based workstations.

So there are quite a few cases where it is more cost effective to use an old and long past its hayday high end video card as a swap device. All the way up to around 2001-2002. From there onwards nearly everything supported sane memory sizes so it is pointless.

Are you looking at the right timings?

[arivanov]

Err... Which hdparm timings are you looking at?

One of the biggest advantages of using VRAM for disks is the nearly 0 seek latency.

As a result even if the card is slower than disk on read you are still likely to have an overall performance gain.

In addition to that there is a number of architectural vagaries to consider. AGP is asymmetric. Reading is considerably slower than writing (can't find anywhere by how much. Damn...).

Re:Just misinformed

[arth1]

If you can buy enough memory not to have to swap, why would you? Swap is for people who can't afford any more memory, and are willing to take a massive performance hit to avoid said expense.

Bzzt - wrong.
Even high-end systems use swap space, because it allows for swapping out parts of memory that isn't called, freeing up that memory for things like disk cache, which does have a positive effect.

Doing "free" on a system here, I see that there's 886492 kB of free memory, of which 879896 kB is used for disk cache. 72892 kB is swapped to disk, and if there were no swap, the disk cache would have been that much smaller. Even if I had umpteen gigabytes of RAM free, that still would be 70 MB of extra cache by using a swap partition. That's a Good Thing.

What's a Bad Thing is when swap is used because you run low on memory -- then you get trashing and a seriously slow system. But on a healthy system with enough free memory, where the kernel can swap out pages not because it has to, but because it makes sense, using swap is a Good Thing.

Re:Built in still uses the bus....

[sumdumass]

This sort of brings up another issue he might be happening. A lot of on board video cards use system memory to function properly. If the swap space is actually in system memory, the extra transfer overhear of going up the bus to the controller that then send it back to the memory that is actually sending the stuff to the video ram.. Well, you see what I mean. The extra few steps could be enough latency per read write operation to slow the thing down compared to a direct access method that would be present with an IDE connection as well as video memory built on to the video car itself.

I think the differences might be as noticeable as turning DMA (direct memory access) on and off. And yes, you can see a big bit of difference. It was actually worth me buying new drives just to have DMA access when it first started becoming available. I remember earlier versions of windows 98 and (95 I think), that wouldn't turn it on by default. After making sure the drives supported it and enabling it, people would almost think they had a new computer. There was that much of a difference in performance.